Two α-benzylidene-γ-butyrolactones, α-(3, 5-dimethyl-4-hydroxybenzylidene)-γ-butyrolactone and α-(3, 5-di-tert-butyl-4-hydroxybenzylidene)-γ-butyrolactone (KME-4), were found to have platelet aggregation inhibitory activity; the latter also had potent antiinflammatory activity and inhibited not only prostaglandin synthetase (PGS) but also 5-lipoxygenase. Further α-benzylidene-γ-butyrolactones were synthesized, and tested for antiinflammatory activity in carrageenin-induced rat paw edema assay (CPE) and for PGS inhibitory activity. It was found that the structural combination of a tert-butyl group at the 3 position, an alkyl group at the 5 position and an oxygen atom at the ^Δ position in α-benzylidene-γ-butyrolactone is necessary for antiinflammatory activity, and that rather broad structural variation is possible for inhibitors of PGS. The structural requirements for antiinflammatory activity in the CPE assay also seem to be partial requirements for inhibitory activity against PGS.